This invention relates to lubricating oil compositions, especially lubricants used in internal combustion engines, comprising base oils high in saturates which are resistant to oxidation.
Lubricating oil compositions are used for the smooth operation of internal combustion engines, power transmission components including automatic transmissions, shock absorbers and power steering devices and gears. The engine oils for internal combustion engines in particular serve to (i) lubricate various sliding interfaces eg between the piston ring and cylinder liner, in bearings of the crank shaft and the connecting rod, and in the valve driving mechanism including cams and valve lifters, (ii) cool the engine, (iii) clean and disperse the combustion products and (iv) prevent corrosion and consequent rust formation. The stringent requirements for high performance engines in recent years has meant greater demand from lubricants used in such engines. Lubricating oils used in such engines usually deteriorate due to oxidation by oxygen and nitrogen oxides (NOx) formed during combustion of fuels and lubricants and that contained in blow-by gas in turn formed by leakage of combustion gases into the crankcase via the piston and cylinder interface. The concentration of NOx increases in the blow-by gas with increasing demand in performance of the engine. The deleterious effects of oxidation can be and have been mitigated by the use of various additives including antioxidants, anti-wear agents, ash-free detergent dispersants, friction modifiers and the like.
Hitherto these have been mitigated to some extent by the use of lubricating compositions which comprise a Group I base oil which is relatively low in saturated hydrocarbons (hereafter xe2x80x9csaturatesxe2x80x9d) in spite of its relatively high propensity to oxidation. Whilst this has meant that the base oil itself is relatively inexpensive, it has had to be supplemented with relatively large amounts of additives/antioxidants to achieve the desired performance. However, by using a relatively more refined feedstock such as the Group II and Group III basestocks high in saturates, it is possible to achieve the desired performance without unduly supplementing the additives/antioxidants used.
It has now been found that by using a specific combination of antioxidants, it is possible to use Group II and Group III base stocks high in saturates with enhanced performance in respect of oxidation stability and fuel efficiency.
Accordingly, the present invention is a lubricating oil composition comprising a base stock and an antioxidant comprising an oil soluble trinuclear organomolybdenum compound of the generic formula:
xe2x80x83Mo3Sxxe2x80x94(Q)xe2x80x83xe2x80x83(I)
wherein x is from 4 to 10, preferably 7, and Q is a core group, which may be a ligand, and at least one other compound selected from a phenolic and an aminic compound characterised in that the base stock has a kinematic viscosity at 100xc2x0 C. (KV100) from about 2 cSt to 20 cSt (2xc3x9710xe2x88x926 to 20xc3x9710xe2x88x926 m2/sec) and a saturates content of at least 85%.
The lubricating oil compositions of the present invention are those that comprise a major amount of a Group II or Group III base stock which may be a natural or synthetic lubricating oil having a KV100 of 2-20 cSt, preferably from 2-12 cSt and a saturates content of at least 85%, preferably at least 88%. Specific examples of Group II basestock high in saturates include inter alia RLOP 500R and Mobil Jurong 500N (with  greater than 97% saturates), and MXT 5 (with 92% saturates); and examples of Group III basestock include inter alia Yubase 4 (with saturate contents of 99.5%) and Yubase 6 (with saturate contents of 97.5%).
According to a further embodiment, the present invention is a method of stabilizing a lubricant composition against oxidative degradation, said composition comprising a base stock which has a kinematic viscosity at 100xc2x0 C. (KV100) from about 2 cSt to 20 cSt (2xc3x9710xe2x88x926 to 20xc3x9710xe2x88x926 m2/sec) and a saturates content of at least 85% said method comprising adding to the basestock an effective amount of an antioxidant comprising an oil soluble trinuclear organomolybdenum compound of the generic formula:
Mo3Sxxe2x80x94(Q)xe2x80x83xe2x80x83(I)
wherein x is from 4 to 10, preferably 7, and Q is a core group, which may be a ligand, and at least one other compound selected from a phenolic and an aminic compound.
The trinuclear molybdenum compounds are of formula (I)
Mo3Sxxe2x80x94(Q)xe2x80x83xe2x80x83(I)
wherein x is from 4 to 10, preferably 7, and Q is a core group. These compounds are relatively new and are claimed and described in our prior published U.S. Pat. No. 5,906,968. The matter disclosed in this prior U.S. patent on the structure, preparation and properties of the trinuclear molybdenum compounds is incorporated herein by reference. In these compounds the core group (Q) may be a ligand capable of rendering the organomolybdenum compound of formula (I) oil soluble and ensuring that said molybdenum compound is substantially charge neutral. The core group (Q) is generally associated with suitable ligands such as L, wherein L is the ligand and y is of a sufficient number, type and charge to render the compound of formula (I) oil soluble and to neutralise the charge on the compound of formula (I) as a whole. Thus, more specifically, the trinuclear molybdenum compound used in the compositions of the present invention may be represented by the formula (II):
Mo3SxLyxe2x80x83xe2x80x83(II)
The ligands xe2x80x9cLxe2x80x9d are suitably dihydrocarbyl dithiocarbamates of the structure (xe2x80x94S2CNR2) wherein the dihydrocarbyl groups, R2 impart oil solubility to the molybdenum compound. In this instance, the term xe2x80x9chydrocarbylxe2x80x9d denotes a substituent having carbon atoms directly attached to the remainder of the ligand and is predominantly hydrocarbyl in character within the context of this invention. Such substituents include the following:
(1) hydrocarbon substituents, ie, aliphatic (for example alkyl or alkenyl), alicyclic (for example cycloalkyl or cycloalkenyl), aromatic-, aliphatic- and alicyclic-substituted aromatic nuclei and the like, as well as cyclic substituents wherein the ring is completed through another portion of the ligand (that is, any two indicated substituents may together form an alicyclic group);
(2) substituted hydrocarbon substituents, ie, those containing nonhydrocarbon groups which, in the context of this invention, do not alter the predominantly hydrocarbyl character of the substituent. Those skilled in the art will be aware of suitable groups (eg halo (especially chloro), amino, alkoxyl, mercapto, alkylmercapto, nitro, nitroso, sulphoxy etc.); and
(3) hetero substituents, ie, substituents which, while predominantly hydrocarbon in character within the context of this invention, contain atoms other than carbon present in a chain or ring otherwise composed of carbon atoms.
The hydrocarbyl groups are preferably alkyl (e.g. in which the carbon atom attached to the remainder of the ligand xe2x80x9cLxe2x80x9d is primary, secondary or tertiary), aryl, substituted aryl and/or ether groups.
Importantly, the hydrocarbyl groups of the ligands should be such that they have a sufficient number of carbon atoms to render the compound (I) soluble or dispersible in the oil to which the trinuclear organomolybdenum compound containing the ligand is added. The total number of carbon atoms present among all of the hydrocarbyl groups of the organomolybdenum compounds"" ligands is suitably at least 21, preferably at least 25, more preferably at least 30 and even more preferably at least 35, typically e.g., 21 to 800. For instance, the number of carbon atoms in each hydrocarbyl group will generally range from 1 to 100, preferably from 1 to 40 and more preferably from 3 to 20.
The antioxidant in the compositions of the present invention suitably also include at least one other compound selected from a phenolic compound and an aminic compound. Among the phenolic compounds, hindered phenols are preferred.
Examples of such phenolic compounds include inter alia:
4,4xe2x80x2-methylene bis(2,6-di-tert-butylphenol)
4,4xe2x80x2-bis(2,6-di-tert-butylphenol)
4,4xe2x80x2-bis(2-methyl-6-tert-butylphenol)
2,2xe2x80x2-methylene bis(4-ethyl-6-tert-butylphenol)
2,2xe2x80x2-methylene bis(4-methyl-6-tert-butylphenol)
4,4xe2x80x2-butylidene bis(3-methyl-6-tert-butylphenol)
4,4xe2x80x2-isopropylidene bis(2,6-di-tert-butylphenol)
2,2xe2x80x2-methylene bis(4-methyl-6-nonylphenol)
2,2xe2x80x2-isobutylidene bis(4,6-dimethyl phenol)
2,2xe2x80x2-methylene bis(4-methyl-6-cyclohexylphenol)
2,6-di-tert-butyl-4-methylphenol
2,6-di-tert-butyl-4-ethylphenol and
2,4-dimethyl-6-tert-butylphenol
Specific hindered phenols which are preferred as the antioxidants may be represented by the generic formulae (III)-(IV) below in which R1, R2, and R3 are the same or different alkyl groups from 3-9 carbon atoms and x and y are integers from 1 to 4. 
Suitable aminic compounds for use in the compositions of the present invention are diaryl amines, aryl naphthyl amines and alkyl derivatives of diaryl amines and the aryl naphthyl amines. Preferred aminic antioxidants are represented by the formulae (VII) and (VIII) wherein each of R4 and R5 is a hydrogen atom or represents the same or different alkyl groups from 1-8 carbon atoms. 
Specific examples of the aminic compounds that may be used in the compositions of the present invention include inter alia:
Monoalkyldiphenyl amines such as eg monooctyldiphenyl amine and monononyl diphenyl amine; dialkyldiphenyl amines such as eg 4,4xe2x80x2-dibutyldiphenyl amine, 4,4xe2x80x2-dipentyldiphenyl amine, 4,4xe2x80x2-dihexyldiphenyl amine, 4,4xe2x80x2-diheptyldiphenyl amine, 4,4xe2x80x2-dioctyldiphenyl amine and 4,4xe2x80x2-dinonyldiphenyl amine; polyalkyldiphenyl amines such as eg tetra-butyldiphenyl amine, tetra-hexyldiphenyl amine, tetra-octyldiphenyl amine and tetra-nonyldiphenyl amine; the naphthylamines such as eg xcex1-naphthylamine and phenyl-xcex1-naphthylamine; butylpheny-xcex1-naphthylamine, pentylphenyl-xcex1-naphthylamine, hexylphenyl-xcex1-naphthylamine, heptylphenyl-xcex1-naphthylamine, octylphenyl-xcex1-naphthylamine and nonylphenyl-xcex1-naphthylamine. Of these, dialkyldiphenyl amine and naphthylamines are preferable.
In general, the antioxidant which comprises the organomolybdenum compound in combination with a phenolic and/or an aminic compound will form a minor component of the total lubricant composition. For example, the organomolybdenum compound typically will comprise about 0.05 to about 5.00 wt % of the total composition, preferably from 0.05 to 2.0 wt %, and more preferably from 0.1 to 0.7 wt %, i.e., the molybdenum metal is suitably present in an amount of from about 25 to 2500 ppm, preferably from about 50 to 1000 ppm, and more preferably from 100 to 700 ppm, and the phenolic and/or aminic compounds about 0.10 to about 3.0 wt % of the total composition.
It has also been found that if the weight ratio of organomolybdenum compound to the phenolic and/or aminic compound in the antioxidant is in the range of about 80:20 to about 20:80, optimum dispersancy retention can be achieved by these combined antioxidants of the present invention.
It is particularly preferred that the antioxidant comprises in addition to the organo molybdenum compound, a mixture of the phenols (III)-(VI) above and the diaryl amines (VII)-(VIII) in a weight ratio ranging from about 80:10:10 to about 10:30:60 respectively, preferably typically 50:15:35 respectively.
Optionally, the antioxidants may be combined with a carrier liquid in the form of a concentrate. The concentration of the combined antioxidants in the concentrate may vary from 1 to 80% by weight, and will preferably be in the range of 5 to 50% by weight.
The antioxidant combination of the present invention can be used with any of the conventional dispersants used hitherto in the lubricating compositions. Examples of such dispersants include inter alia the polyalkylene succinimides, Mannich condensation products of polylalkylphenol-formaldehyde polyamine and boronated derivatives thereof. However, it is preferable to use ashless dispersants such as the ashless succinimides, especially the polyisobutenyl succinimides of a polyamine such as eg tetraethylenepentamine or its homologues, benzylamine ashless dispersants, and ester ashless dispersants. The dispersants are generally used in the compositions of the present invention in an amount ranging from about 2-10% by weight based on the total weight of the lubricant composition, preferably from about 4-8% by weight.
A feature of the present invention is that the compositions of the present invention the presence of a trinuclear organo molybdenum compound facilitates the control of deposit formation from engine oils. More specifically, formulations containing eg a contribution of 200-750 ppm, preferably from 450-550 ppm of Mo metal from the trimer and an additional contribution of from 80-100 ppm from a detergent inhibitor package enables the amount of deposit formed to be significantly reduced. Adverse effects, if any, due the presence of such molybdenum compounds in the formulation, eg copper strip corrosion, are readily mitigated by including in the formulation a corrosion inhibitor or a metal passivator. This reduction in deposit formation has been monitored by the so-called TEOST-MHT-2 test which test is similar to the conventionally used TEOST-33 test method except that it is run at a relatively lower temperature and for a longer time. These tests are especially designed to test the formulations for a GF-3 specification. The TEOST-33 test is carried out at temperature cycles which fluctuate from 200-500xc2x0 C. and last for about 2 hours and results in bulk oxidation of the oil (about 100 g). In contrast, the TEOST-MHT-2 test relates to high temperature engine deposits as measured in tests such as TU3HT and is carried out at about 285xc2x0 C. over 24 hours and is a thin-film test on about 8 g of oil. The TEOST-MHT-2 test measures deposits produced on a heated rod or in the oil itself (filtered residue) and the GF-3 specification is envisioned to specify a limit of 40 mg deposit. From the results in the Examples below it will be seen that the presence of a trinuclear organo molybdenum compound in such oils results in about 66% reduction in the total weight of the deposits formed which satisfies the GF-3 specification.
In general, these lubricating compositions may include additives commonly used in lubricating oils especially crankcase lubricants, such as antiwear agents, detergents, dispersants, rust inhibitors, viscosity index improvers, extreme-pressure agents, friction modifiers, corrosion inhibitors, emulsifying aids, pour point depressants, anti-foams and the like.
A feature of the present invention is that lubricant compositions comprising high saturates base oils and trinuclear organomolybdenum compounds in combination with a phenolic and/or an aminic compound as antioxidant provide unexpected improvement in oxidation control and significant benefits in fuel economy. In the case of lubricants compositions comprising high saturates base oils for diesel engine oils, the present invention confers the added benefits of viscosity increase control and dispersancy retention over compositions which contain only one of these antioxidants used alone.
The present invention is further illustrated with reference to the following Examples: